The goal of these studies is to determine whether adult human thymuses remain active and capable of generating new T cells in drug-treated HIV- infected individuals. Experiments detailed in this application are designed 1) to determine the potential of adult-derived human thymic stroma and CD34+ cells to support thymopoiesis in vivo in the SCID-hu mouse and to identify means of enhancing that potential, and 2) to assess the impact of HIV-1 on the ability of human adult thymic stroma and CD34+ cells to support thymopoiesis in vivo.